The present invention relates to a rare earth ion doped optical element used in the fields of light communication, light information processing, light applied instrumentation control and the like to which coherent light is applied, for example, a rare earth ion doped solid state optical element, a rare earth ion doped optical fiber element, a rare earth ion doped laser element and a rare earth ion doped optical amplifying element.
As the first prior art of the rare earth ion doped laser element, there has been known "Continuous-wave 1.5 .mu.m thulium cascade laser" (Optics Letters vol. 16, p. 232, 1991) written by R. C. Stoneman and L. Esterowitz.
FIG. 18 shows the structure of the rare earth ion doped laser element. In FIG. 18, the reference numeral 11 denotes a laser crystal comprised of YLiF.sub.4, the reference numeral 12 denotes an incident portion of the laser crystal 11, the reference numeral 13 denotes an emitting portion of the laser crystal 11, the reference numeral 14 denotes an incident lens, the reference numeral 15 denotes excitation light, the reference numeral 16 denotes an output laser beam, the reference numeral 17 denotes a pump laser, and the reference numeral 18 denotes an emitting lens.
FIG. 19 shows the energy level transition of Tm (Thulium) ion when doped in the host laser crystal YLiF.sub.4. Tm in this case is referred to as the activator ion. In FIG. 19, the reference numeral 21 denotes energy level transition caused by light absorption, the reference numeral 25 denotes energy level transition caused by phonon emission, the reference numeral 24 denotes energy level transition caused by light radiation, and the reference numerals 27, 28, 29 and 30 denote energy levels. An axis of ordinates 26 denotes energy with a unit of cm.sup.-1 (kayser).
As shown in FIG. 18, the excitation light 15 having a wavelength of 795 nm emitted from the pump laser 17 is converged by the incident lens 14. The excitation light 15 thus converged is radiated onto the laser crystal 11 through the incident portion 12. In this case, the energy state of the Tm ion is raised from the ground state energy level 27 to the high energy level 28 by energy absorption as shown in the level transition 21. Then, the energy state of the Tm ion is lowered from the level 28 to the level 29 by phonon emission as shown in the level transition 25. Thereafter, the energy state of the Tm ion is lowered from the level 29 to the level 30 as shown in the level transition 24. When the level transition 24 is carried out, light having a wavelength of 1.5 .mu.m is radiated. The level 30 is referred to as the terminal or lower level. The light thus radiated is caused to resonate by reflecting films provided on the incident portion 12 and the emitting portion 13, and is then emitted as a laser beam having a wavelength of 1.5 .mu.m from the emitting portion 13.
As the second prior art of a rare earth ion doped laser element, there has been known "Amplification and Lasing at 1.3 .mu.m in Praseodymium-doped Fluorozirconate Fibres" (Electronics Letters vol. 27, P. 626, 1991) written by Y. Durteste, M. Monerie, J. Y. Allain, and H. Poignant.
FIG. 20 shows the structure of a rare earth ion doped laser element. In FIG. 20, the reference numeral 51 denotes an optical fiber, the reference numeral 52 denotes an incident portion of the optical fiber 51, the reference numeral 53 denotes an emitting portion of the optical fiber 51, the reference numeral 54 denotes an incident lens, the reference numeral 58 denotes an emitting lens, the reference numeral 55 denotes excitation light, the reference numeral 56 denotes an output laser beam, and the reference numeral 57 denotes a pump laser.
FIG. 21 shows the energy level transition of Pr (Praseodymium) ion, i.e., Pr is in this case the activator ion, with which the optical fiber 51 is doped. In FIG. 21, the reference numeral 61 denotes level transition caused by light absorption, the reference numeral 64 denotes level transition caused by light radiation, and the reference numerals 67, 68 and 70 denote energy levels. An axis of ordinates 66 denotes energy with a unit of cm.sup.-1.
The excitation light 55 having a wavelength of 1.017 .mu.m emitted from the pump laser 57 is radiated onto the optical fiber 51 through the incident portion 52. The excitation light 55 is absorbed by the Pr ion. Consequently, the energy state of the Pr ion is shifted from the ground state energy level 67 to the high energy level 68 as shown in the level transition 61. Then, when the energy state of the Pr ion is shifted from the high energy level 68 to the terminal energy level 70 as shown in the level transition 64, light having a wavelength of 1.31 .mu.m is radiated. The light thus radiated is caused to resonate by reflecting films provided on the incident portion 52 and the emitting portion 53 so that population inversion takes place between the levels 68 and 70. Then, the light is emitted as a laser beam having a wavelength of 1.31 .mu.m from the emitting portion 53.
In the case where an optical amplifier is used, population inversion does not take place between the levels 68 and 70. The light having a wavelength of 1.31 .mu.m which is incident as signal light on the optical fiber is amplified by light which is radiated at the time of transition from the level 68 to the level 70.
Referring to the Tm ion according to the first prior art, light is radiated so that a significant portion of the ions remains at the energy level 30. The reason is that the fluorescence lifetime of the level 30 is very long on the order of 10 ms and there is an energy difference of 5000 cm.sup.-1 or more between the medium energy level 30 and the ground state energy level 27 so that the probability of transition from the level 30 to the level 27 by phonon emission is very small. Thus, the probability of returning to the level 27 is small so that the probability of transition to the high energy level is decreased. As a result, even if the power of the excitation light is increased, the absorption efficiency of the excitation light is lowered so that the output laser beams are reduced. Referring to a conventional 4-level laser element, the efficiency of the output light to the input light is low, i.e., about 13%.
Referring to the Pr ion according to the second prior art, light is radiated so that the energy level stays at the level 70. The reason is that the fluorescence lifetime of the level 70 is long and there is an energy difference of about 2000 cm.sup.-1 between the medium energy level 70 and the ground state energy level 67 so that the probability of transition from the level 70 to the level 67 by phonon emission is small. Thus, the probability of returning to the level 67 is small so that the probability of transition to the high energy level is decreased. As a result, even if the power of the excitation light is increased, the absorption efficiency of the excitation light is lowered so that the output laser beams are reduced. Referring to the laser element according to the second prior art, the efficiency of the emitted light to the incident light is about 30%.
Referring to the above-mentioned rare earth ion doped laser elements, accordingly, it is important that the transition from the terminal energy level to the ground state energy level should be carried out efficiently. The number of rare earth ions in a given volume of the host material is limited. Consequently, if the population of the medium energy level is significant with respect to the population of the ground state energy level, the transition to the high energy level is decreased. Consequently, the absorption efficiency of the excitation light is lowered and high output is not obtained.
There has been proposed a method for cooling the laser element down to 7K. However, it is very difficult to execute this method in respect of practicality in an industrial environment.